Various types of pumps are used to pressurize fluid at high pressure. As a driving source for these high pressure pumps, motor-driven system, hydraulic booster system, pneumatic booster system, etc. are known.
A representative example of direct-coupled motor type system is a three-throw plunger pump as commonly used. In this type of pump, it is necessary to mount a large speed reducing gear on crankshaft for the control of number of revolutions and for increasing output of the motor. Even in such case, it is difficult to reduce the speed to less than 400 rpm, and upper pressure limit is about 1500 kgf/cm.sup.2. From the reason of mechanism, it is impossible to eliminate liquid trap, and it is practically impossible to perform processing of different liquid phases by a single pump. Also, in case high pressure circuit is closed from some reason, pressure may be infinitely increased, and this means that it is necessary to provide a safety valve and to frequently confirm its reliability.
In the hydraulic booster system, hydraulic pump is operated by an electric motor, and a booster pump based on Pascal's principle is driven by the hydraulic pressure to obtain the high pressure as required. However, the system itself must be designed in large size because hydraulic pump, hydraulic valve, hydraulic tank, etc. are required. Also, energy efficiency is decreased because electric energy is converted to hydraulic pressure by motor and hydraulic pump, and this energy is used. Further, it is not possible to perform pressure control below the level of "the lowest hydraulic pressure generated x booster ratio". Because oil temperature is varied due to the change of ambient temperature, fine adjustment of hydraulic pressure must be carried out.
In the pneumatic pressure booster system, the required pressure is attained by driving a booster pump by compressed air based on Pascal's principle. In general, however, pneumatic pressure of 10 kgf/cm.sup.2 is used because of restriction by high pressure gas law. Therefore, in case it is wanted to attain high pressure, e.g. in case it is wanted to attain the pressure of 2000 kgf/cm.sup.2, booster ratio must be 200-fold. Because higher booster ratio is required, a large quantity of air is needed, and this means that a very large air compressor must be provided. Also, a dryer must be arranged because moisture components contained in the air must be removed, and this leads to still larger size of the system. Because it is not possible to reduce the pressure below the level of booster ratio in this case, even when this pump is operated at the lowest pressure of 0.5 kgf/cm.sup.2, it is not possible to operate at 100 kgf/cm.sup.2 or less. Because electric energy is converted to pneumatic pressure by motor and air compressor and this energy is utilized, the energy efficiency is low.
As described above, none of the conventional type high pressure pumps used for the purpose of pressurizing fluid at high pressure meets the requirements such as lightweight and compact design, improvement of energy efficiency, accuracy of the generated pressure in the required pressure range from low pressure to high pressure, or high reliability operation.